Most water contains alkaline earth metal cations, such as barium, strontium, calcium and magnesium, and anions, such as sulfate, bicarbonate, carbonate, oxalate, phosphate, silicate and fluoride. When combinations of these anions and cations are present in concentrations which exceed the solubility of their reaction products, precipitates form until their product solubility concentrations are no longer exceeded. For example, when the barium ion and sulfate ion exceed the solubility of the barium sulfate reaction product, a solid phase of barium sulfate will form as a precipitate.
Solubility product concentrations are exceeded for various reasons, such as evaporation of the water phase, change in pH, pressure or temperature and the introduction of additional ions which can form insoluble compounds with the ions already present in the solution.
As these reaction products precipitate on the surfaces of the water-carrying or water-containing system, they form adherent deposits or scale. The scale prevents effective heat transfer, interferes with fluid flow, facilitates corrosive processes, and harbors bacteria. Scale is an expensive problem in many industrial water systems, in production systems for oil and gas, in pulp and paper mill systems, and in other systems, causing delays and shutdowns for cleaning and removal.
Once through and recirculating, cooling water systems are subject to the formation of scale deposits. Waterside problems encountered in boilers and steam systems include the formation of scale and other deposits, corrosion and foam. Scale and other deposits on heat-transfer surfaces can cause loss of the thermal efficiency of the boiler and can make the temperature of the boiler metal increase. Under scaling conditions, temperatures may go high enough to lead to failure of the metal due to overheating.
Barium and strontium sulfate scale deposits present a unique and sometimes "unsolvable" problem. Under most conditions, these sulfates are considerably less soluble in all solvents than any of the other commonly encountered scale-forming compounds. It is generally acknowledged that barium sulfate scale is almost impossible to remove by chemical means. Consequently, barium sulfate must be removed mechanically or the equipment, pipes, etc., containing the deposit must be discarded.
The incidence of barium sulfate scale is worldwide, and it occurs principally in systems handling subsurface waters. The barium sulfate scale problem is of particular concern to the petroleum industry since increasing volumes of water are produced with petroleum and more petroleum is produced by the waterflooding method of secondary recovery. The scale may occur in many different places, including production tubing, well bore perforations, the area near the well bore, gathering lines, meters, valves and in other production equipment.
Deposition of scale in production facilities and formation channels is a well-known source of problems in oil recovery. Barium sulfate scale is particularly troublesome when sulphate-rich seawater is used as an injection fluid in oil wells whose formation water is rich in barium ions. This scale causes severe problems in U.S. oil fields and older North Sea oil fields. Scaling of this nature is expected to occur during advanced production stages in other North Sea fields particularly after seawater breakthrough has taken place.
Barium sulfate scale may also form within subterranean formations such as in disposal wells. Scales and deposits can be formed to such an extent that the permeability of the formation is impaired resulting in lower flow rates, higher pump pressures, and ultimately abandonment of the well.
U.S. Pat. No. 4,708,805, issued to D'Muhala on Nov. 24, 1987, discloses a process and compositions for the sequestration of barium sulfate scale. The compositions comprised an aqueous solution of citric acid, a polycarboxylic acid, and an alkylene-polyaminopolycarboxylic acid. The preferred aqueous sequestering solution had a pH in the range of about 9.5 to about 14. The pH was provided by a base selected from the group consisting of potassium hydroxide, potassium carbonate, and mixtures thereof. This patent is hereby incorporated by reference herein.
De Jong et al. in U.S. Pat. No. 4,190,462 disclose that barium sulfate scale can be removed from remote locations extending into a subterranean earth formation by contacting the scale with an aqueous solution consisting essentially of water, a monovalent cation salt of a monocyclic macroyclic polyamine containing at least two nitrogen-linked carboxymethyl groups and enough monovalent basic compound to provide a solution pH of about 8.
Although the prior art compositions remove scale, the rate of dissolution is slow and the amount of scale dissolved is small. Therefore, what is needed is a method and composition for removing barium scale at a substantially increased rate and a high capacity.